The invention relates to fiber optic telecommunications and more particularly to long-haul telecommunications.
When they propagate in fibers, optical signals are subject to various physical phenomena that degrade signal quality, mainly line losses, accumulation of noise and Gordon-Haus jitter.
Line losses can be compensated by in-line optical amplification, for example using an erbium-doped fiber amplifier (EDFA).
However, in-line amplification cannot prevent Gordon-Haus jitter, which causes uncertainty in the time of arrival of the bits of the signal.
What is more, in-line amplification leads to accumulation of noise caused by spontaneous emission in the optical amplifiers.
Also, non-linear effects such as the Kerr effect, the Raman effect and crossed-phase modulation (XPM) in wave-length division multiplex (WDM) transmission systems limit the maximum transmission distance of optical signals in some cases.
This is why it is necessary to provide regenerators at regular intervals in fiber optic transmission lines, preferably xe2x80x9c3Rxe2x80x9d regenerators which Retime the optical signals, Reshape them to restore their original envelope and Re-amplify them to their original amplitude levels.
This kind of regenerator is described in French application FR 9809410 in the name of the Applicant, for example, in the document xe2x80x9c140 Gbit/s polarization-insensitive and wavelength-independent InP Mach-Zehnder modulator for all optical regenerationxe2x80x9d, Electronics Letters, Vol.35, No.9, p.730, Apr. 29, 1999, and in xe2x80x9cContribution à l""xc3xa9tude de la rxc3xa9gxc3xa9nxc3xa9ration optique dans les systxc3xa8mes de transmission par solitonsxe2x80x9d [A contribution to studying optical regeneration in soliton transmission systems], by O. Leclerc, a doctoral thesis submitted to Nice-Sophia-Antipolis University on Oct. 1, 1998, all of which documents are incorporated in the present description by reference.
The regenerator described in the above documents uses a Mach-Zehnder modulator for synchronous phase/amplitude modulation of the optical signals to be regenerated. Each arm of the Mach-Zehnder modulator includes a semiconductor optical amplifier whose gain is modulated using a recovered clock signal.
To this end, in one possible configuration, the regenerator uses an all-optical or an opto-electronic circuit to recover the clock signal from the optical signals to be regenerated.
However, in the above prior art regenerator, the interferometer structure and the clock signal recovery circuit constitute separate and independent entities, which impedes integration of the component.
The present invention aims to improve on a regenerator of the type disclosed in the above documents by proposing a regenerator having greater aptitude for integration.
To this end, the invention provides a device for regenerating optical signals by intensity modulation and phase modulation of the optical signals, said device including:
a mode locking laser for recovering a clock signal representing the clock of the bits of the optical signals to be regenerated, and
a modulation interferometer structure having a first interferometer arm and a second interferometer arm, enabling amplitude modulation and phase modulation of optical signals by said clock signal, and including a semiconductor amplifier in the first interferometer arm,
wherein the mode locking laser is coupled to said first interferometer arm so that said amplifier of the first interferometer arm is a common component of the mode locking laser and said interferometer structure.
The device of the invention can also have one or more of the following features:
said amplifier of the first arm is chosen so that the modulation of its gain caused by the effects of saturation by the optical signal to be regenerated triggers mode locking of said mode locking laser at a wavelength inside the emission band of the amplifier of the first interferometer arm,
there is a second amplifier in the second interferometer arm and it includes a second mode locking laser for recovering a second optical clock signal also representing the clock of the bits of the optical signals to be regenerated and having a second wavelength different from the wavelength of the clock signal of the first mode locking laser, the second mode locking laser being coupled to said second interferometer arm so that said amplifier of the second interferometer arm is a common component of the second mode locking laser and said interferometer structure,
said second semiconductor optical amplifier of the second interferometer arm is substantially identical to the first semiconductor optical amplifier of the first interferometer arm,
said interferometer structure is a Michelson structure and it further includes an optical circulator having a first port receiving the optical signals to be regenerated, a second port connected to the interferometer input/output port of the Michelson structure, and a third port which is an output port for the regenerated optical signals,
the mode locking laser is a ring laser including:
a fiber for recovering the clock signals whose input is coupled to an output fiber connected to the third port of said optical circulator and whose output is connected to first coupling means for coupling optical clock signals into the first arm of said interferometer structure,
said first interferometer arm with said first optical amplifier,
a connecting fiber for connecting the interferometer input/output of the interferometer structure to the second port to the optical circulator, and
a selector filter whose center wavelength is the wavelength of the optical clock signals, disposed in said recovery fiber or in said first coupling means,
the first arm of said interferometer structure has an end which is reflective for the optical signals to be regenerated and at least partially transparent for optical signals at the wavelength of the recovered clock signals, and the mode locking laser is a ring laser including:
a fiber for recovering the clock signals, one end of which is coupled to said end of the first arm and whose other end is connected to first coupling means for coupling optical clock signals into the first arm of said interferometer structure,
said first interferometer arm with said first optical amplifier, and
a selector filter whose center wavelength is the wavelength of the optical clock signals, disposed in said recovery fiber or in said first coupling means,
it further includes second coupling means for coupling recovered optical clock signals into the second arm of the interferometer structure,
there is an optical isolator in said recovery fiber to impose the direction of propagation of the recovered clock signals,
there are means for time storage of the clock signals in said recovery fiber to smooth the various spectral contributions of the recovered optical clock signals,
said storage means include an optical fiber of predefined length,
there is an additional semiconductor optical amplifier in said first coupling means,
the mode locking laser is a resonant cavity laser oscillator including:
the first arm of said interferometer structure and its end, which is reflective for the optical signals to be regenerated and for the recovered optical clock signals,
a lateral branch of said interferometer structure, one end of which is coupled to said first arm and whose other end is reflective for the optical clock signals, and
a selector filter in said lateral branch whose center wavelength is the wavelength of the optical clock signals,
said selector filter is a Bragg reflector,
there is a phase area in said lateral branch for adjusting the optical length of the cavity,
there is an additional semiconductor optical amplifier in said lateral branch,
the interferometer structure and the mode locking laser are integrated on the same substrate,
said interferometer structure is a Mach-Zehnder structure,
said mode locking laser is a ring laser including:
a fiber for recovering the clock signals whose input is coupled to first coupling means for sampling the clock signals from said first arm and whose output is connected to second coupling means for feeding optical clock signals propagating in the recovery fiber into the first arm,
an optical isolator for imposing the direction of propagation of the recovered clock signals, and
a selector filter whose center wavelength is the wavelength of the optical clock signals,
the direction of propagation of the clock signals imposed by the optical isolator is a contrapropagating direction relative to the signals to be regenerated,
the mode locking laser is a resonant cavity laser oscillator including:
a first lateral branch of said interferometer structure, one end of which is totally reflective for the optical clock signals and whose other end is coupled to said first arm,
said first arm of said interferometer structure, defining the central part of the cavity, and
a second lateral branch, one end of which is coupled to said first arm and whose other end is totally reflective for the optical clock signals,
said interferometer structure and the mode locking laser are implemented as integrated components on the same substrate,
it includes a fiber for coupling a part of the power of the regenerated optical clock signals into the second arm of said interferometer structure, and
said interferometer structure is an integrated component.